Process for the conversion of hydrogen



Unite States PROCESS FOR THE CONVERSION OF HYDROGEN N Drawing. FiledDec. 20, 1956, Ser. No. 629,471

14 Claims. (Cl. 23-.-210) The invention described herein may bemanufactured and used by or for the Government for governmental purposeswithout payment to us of any royalty thereon.

This invention relates to catalysis and, more particularly, to thecatalytic reorientation of orthoand parahydrogen.

Hydrogen is, of course, a biatomic molecule and it has been known forsome time that this molecule exists in two forms: namely, ortho-hydrogenand para-hydrogen. These two forms are characterized and diflerentiatedfrom one another by the relationship that exists within the nuclei ofthe atoms that form the molecule. Each of the atoms has within itsnucleus a force or field which can be mathematically described as thoughcaused by a charged particle spinning around in a definite orbitalpattern; however, the relationship between these orbital patterns ornuclear spins of the two atoms that form the hydrogen molecule may bedifferent as the particles may be described as rotating either in thesame direction or in opposite directions. If the particles that make upthe nuclei of the two atoms are rotating in the same direction, themolecule is identified as an ortho-hydrogen molecule; whereas, if theparticles in the nuclei are orbiting in directions opposite to oneanother, the molecule is denominated a para-hydrogen molecule. At thispoint it should be mentioned that the spin orbits of the two atoms in agiven molecule of hydrogen are always parallel although they may rotatein different directions, as aforementioned.

Ortho-hydrogen and para-hydrogen molecules co-exist with one another andhave a definite equilibrium relation at a given temperature wherein afixed ratio of orthoand para-hydrogen is present; however, this ratiovaries Widely at different temperatures. For example, at roomtemperature so-called normal hydrogen at equilibrium contains 75%ortho-hydrogen and 25% para-hydrogen; whereas, the equilibrium state ofhydrogen at 20 K. is such that 99.8% para-hydrogen and only 0.2%ortho-hydrogen is present.

Now, this fact becomes extremely important in the liquefaction ofhydrogen for the following reasons. If we start with normal gaseoushydrogen (75 ortho-) at room temperature and, of course, at equilibrium,it will be in an unstable energy condition when it reaches thetemperature of 20 K. at which it liquefies if the liquefaction iscarried out rapidly in the customary manner. The hydrogen will,therefore, change or convert slowly from the unstable, high energy stateto the stable, low energy level of equilibrium at 20 K. This conversionor reorientation of the nuclei of the ortho-molecules to para-moleculesuntil 99.8% para-hydrogen is present is a highly exothermic energyreaction that causes a substantial volume of the liquid hydrogen to boiloff due to the heat liberated. For example, tests have shown that ifnormal hydrogen (75% ortho-) is quickly liquefied so that little or noreorientation or conversion takes place during the liquefaction processand the liquid hydrogen is stored in a perfectly insulated vessel atabout 20 K., nearly one-fifth of the original liquid volume is lost andconverted to the gaseous state during the first 24-hour period due tothe exothermic energy reaction that takes place as the hydrogen slowlychanges to the equilibrium energy condition. It will become apparent,therefore, that the liquid hydrogen must be stored in a state of stableenergy equilibrium if this rapid boiling away and depletion of theliquid is to be prevented. Actually, in accordance with the equilibriumconditions at 20 K., this means converting normal hydrogen tosubstantially pure para-hydrogen either during the liquefaction processor immediately after while low temperature refrigeration is stillavailable to carry off the heat of conversion liberated by theexothermic reorientation energy reaction. Normally, however, thisexothermic energy reaction that reorients the spin-orbits of the atomsin the hydrogen molecules proceeds much more slowly than theliquefaction process by which the hydrogen gas is liquefied. Therefore,it would be highly advantageous to catalyze this reorientation reactionso that the hydrogen would be in a state of stable energy equilibrium atthe time it becomes liquefied or shortly thereafter; or, in other words,have the product of the liquefaction process nearly pure para-hydrogen.

Previous attempts to catalyze the foregoing energy reaction have metwith a measure of success; however, the reaction was still quite slow,large quantities of the catalyst were required and relatively largeliquefier components were needed for a given production rate due to thespace requirements of the catalyst. The most effective of the prior artcatalysts was a bed catalyst placed in the liquefier reservoir andcomposed of Cr O on alumina pellets. The catalyst bed was immobilized byplacing it under a coarse heavy filter. Liquid hydrogen was fed from theexpansion valve of the liquefaction unit, filtered through the catalystbed and, finally, delivered to a storage Dewar through a transfersiphon. Using approximately 65 liters of about 20% Cr O on a bed of /sbyAs-inch alumina pellets, a conversion to 97% para-hydrogen at aproduction rate of 240 liters of liquid hydrogen per hour was achieved.

It has now been found, however, in accordance with the present inventionthat an unsupported ferric oxide gel provides a much more efficient andgreatly improved catalyst for both the ortho-para and para-orthoconversion of hydrogen in either the liquid phase or vapor phasecondition.

The principal object of the present invention is, therefore, theprovision of a new and superior catalytic agent for both the liquid andvapor phase conversion of orthoto para-hydrogen and paratoortho-hydrogen.

Another object of the invention is to provide a catalyst for thereorientation of orthoand para-hydrogen that is approximately fortytimes as efiicient and effective as Cr O on alumina pellets.

A further object of the invention is to provide a novel process for thereorientation of orthoand para-hydrogen to produce an equilibriumcondition.

Other objects of the invention are to provide a hydrogen conversioncatalyst that requires considerably less space in the liquefaction unit,one which is relatively simple to prepare and activate, a catalyst thatis inexpensive and one which can be stored for long periods of timewithout harmful deterioration.

Additional objects of the invention will be in part apparent and in partpointed out specifically hereinafter as the description proceeds.

The ferric oxide gel that comprises the subject matter of the claimedinvention is produced by precipitation from an aqueous solution of asoluble ferric salt with a soluble hydroxide. The resultant precipitateusually takes the form of a brown gelatinous substance composed ofextremely'minute particles although its color may vary from red toalmost black. The low solubility of Fe O in water produces an enormoussupersaturation preceding precipitation which is quite favorable to theformation of a precipitate having minute particles and a large surfacearea. These particles orient themselves into an .enmeshing network thatentrains water and constitutes the gelatinous precipitate. This bulkybrown gel loses water gradually on standing and becomes more compact.The precipitate must be washed with water until most of the anion of theoriginal ferric salt has been removed; however, washing of theprecipitate until approximately 0.1% of the original solution remains,has been found suificient. After the precipitate has been Washed it isfiltered and dried; Thereafter, the dried precipitate is crushed andgraded through sieves to ob tain the desired range in particle size. Inthis form the precipitate may be stored over extended periods inordinary glass jars until needed; Immediately prior to use, however, thecatalyst must be activated by heating it to a temperature of betweenapproximately 110 and 120 0., preferably in a drying oven, for-l2 to 16hours at a pressure of about one mm. Hg or less. The vacuum should bebroken with pure hydrogen gas while the catalyst is still hot.

Repeated tests with ferric oxide gel as a catalytic agent in theconversion of orthoand para-hydrogen have been performed by one of theinventors at the National Bureau of Standards Cryogenic EngineeringLaboratory and have resulted in the conversion of liquid normal hydrogen(75% ortho-) to 5% ortho-hydrogen and less'at rates of approximately2000 S.T.P., liters per minute (2.6 liquid liters per minute) per literof catalyst used, or 156 liquid liters per hour per liter of catalyst.These results show ferric oxide gel to be approximately 40 times aseffective as Cr O on alumina pellets when measured under the sameconditions.

Tests performed by one of the inventors at the aforementioned laboratoryhave also resulted in the conversion of 99.8% para-hydrogen toessentially the equilibrium orthoto para-hydrogen ratio at temperaturesranging from 80 K. to 225 K. The rates of reconversion at temperaturessomewhere between 140? C. and 200 7 C. therefore, 140 C. should beconsidered the maximum activation temperature. 7

The mechanism by which ferric oxide accomplishes the catalyticreorientation of orthoand para-hydrogen is believed to depend upon twounsuspected, but important, properties which will now be described.

The first of these properties is the high magnetic susceptibility of theminute gel particles and the attendant tremendous magnetic field*gradient confined to a space of the order of the size of the moleculeitself. Basically, each ferric oxide gel particle is, in itself, aminute, but powerful magnet. The form of the gel is such that itprovides a tremendous surface area having a very porous cellularstructure giving the hydrogen molecules a large interface with which tocome in contact. The force of the magnetic field around a particle ofthe 7 catalyst diminishes in intensity very rapidly as the dis- 7therefore, act to turn only one of the atoms and cause it ofpara-hydrogen have in. these tests been especially high in thetemperature region of 80 K. to 140 K. For example, at 100 K. a feed of99.8% para-hydrogen was converted to within two percent of theequilibrium (38.5% para-hydrogen) concentration at the rate of about25.0 S.T.P. liters per minute per liter of catalyst. This property,consisting of the ability to catalyze the endothermic reconversion ofpara-hydrogen to ortho-hydrogen, points to an important applicationwhich has not previously been mentioned, i.e., utilization of the paratoortho-conversion as a heat sink or source of refrigeration. Thus aquantity of the hydrous ferric oxide suitably placed in tubes in theinsulation surrounding a container of liquid para-hydrogen could, byparato ortho-conversion of the boil-off para-hydrogen vapor, effectivelycool a radiation shield attached to the tubes and thereby considerablyreduce the heat leak to the remaining liquid in the storage vessel.

Particle size has also been found to have a decided influence on theeffectiveness of the hydrous ferric oxide gel in the ortho-paraconversion. In general, the effectiveness of the catalyst falls offrapidly when the particle size exceeds that which will pass through a20-mesh screen. Samples made up of particles ranging from 30 to SO-mesh,however, exhibited about the same activity as samples composed of 50 toIOU-mesh particles. There seems to be little advantage in using materialfiner than IOO-mesh as problems arise in connection with containment ofthe catalyst and the pressure drop across the bed which ove'rbalancesthe small increase in activity.

At'this point it should be mentioned that the catalyst is not ferrichydroxide as the amount of water in the gel varies over wide limits and,therefore, bears no to flip over and reorient itself into the morestable energy condition. This mechanism is effective to realign thehydrogen atoms from the ortho-relation to the paraor vice versadepending, of course, upon which condition is the more stable energystate.

The second important property of the ferric oxide gel catalyst is itsability to disassociate the hydrogen molecule and adsorb individualatoms on its surface. This property of the catalyst may be referred toas activated adsorption. When disassociation of the molecules takesplace and atoms of hydrogen are adsorbed on the surface of the ferricoxide particles, these adsorbed atoms will re-combine with otherdisassociated atoms in the stable energy relation. Thus, the catalystprovides a second mechanism that is eifective to re-align the spinorbitsof the hydrogen atoms into an equilibrium condition irrespective ofwhether the conversion is ortho-.

para or para-ortho.

The use of any catalyst to effect an equilibrium conversion of orthotopara-hydrogen during the process of liquefaction, of necessity, causes areduction in the production rate of the =liquid hydrogen when comparedwith a liquefaction process in which no attemptis made to reorient themolecules, as part of the refrigeration capacity of the liquefier mustbe used to carry off the exothermic heat of conversion. The claimedcatalyst, however, in so far as the conversion reaction is concerned, ismany times more effective and active to bring about the desiredreorientation than the prior art catalystsfor this purpose. Therefore,the production rate of liquid parahydrogen is primarily limited by therefrigeration capacity of the liquefier and not the catalyst which insmall quantities is sufficient to bring about the conversion morerapidly than the refrigeration unit can carry off the heat iiberatedthereby. The, Cr O on alumina catalyst, on the other hand, must bepresent in relatively large quantities to cause the required conversionat a given production rate and it thus becomes a limiting factor inthose liquefaction units wherein space for the catalyst is limitedbecause, if the unit cannot accommodate the amount of catalyst neededfor the conversion at said production Also, the

rate, the production rate must be reduced to that which can be handledby the catalyst.

The particle size of the catalyst is not a particularly critical factorif maintained within reasonable limits as many different mesh sizes willbring about the desired end results; however, judicious selection of theparticle size will improve the activity and rate of energy reaction andsuch selection will depend to a large extent on the particularliquefaction apparatus used. For example, in equipment wherein a largedrop in pressure across the catalyst bed can be tolerated and stillproduce the desired production rate, an extremely finely divided form ofthe catalyst can be used. On the other hand, if the pressure drop iscritical, a coarser form should be selected even at the sacrifice ofsome catalytic activity. The particle size chosen is also somewhatdependent on whether the conversion is to be carried out in theliquid-phase or vapor-phase as obviously, the vapor-phase permits thehydrogen to permeate the catalyst bed much more easily and thoroughly.The foregoing considerations, however, are well within the skill of theart.

Having thus described the several useful and novel features of thecatalyst and catalytic reaction for the ortho-para and para-orthoconversion of hydrogen, it will be seen that the many useful objects forwhich they were designed have been achieved. We realize, however, thatcertain changes and modifications may occur to those skilled in the artwhich are within the broad teaching of the instant invention; hence, itis our intention that the scope of protection aiforded hereby shall belimited only in so far as said limitations are expressly set forth inthe appended claims.

What is claimed is:

l. The process for catalytically converting hydrogen to an equilibriumenergy state within the temperature range of from about 20 K. to about240 K. which comprises passing the hydrogen through a porous bed ofheatactivated ferric oxide gel particles.

2. A process as set forth in claim 1, wherein the ferric oxide gelparticles are of a size which will pass a 20 mesh screen and theparticles are activated by heating to a temperature within the range offrom about 110 C. to about 140 C.

3. The catalytic process for converting a mixture of ortho-hydrogen andpara-hydrogen in an unstable energy state into an equilibrium energycondition within the temperature range of from about 20 K. to about 240K. which comprises passing the unstable mixture through a porous bed ofheat-activated ferric oxide gel particles.

4. The catalytic process for increasing the ratio of para-hydrogen toortho-hydrogen in a mixture thereof within the temperature range of fromabout 20 K. to about 240 K. which comprises lowering the temperature ofthe mixture, passing said mixture through a finelydivided heat-activatedbed of ferric oxide gel, and removing the exothermic heat of reaction.

5. The catalytic process for increasing the ratio of ortho-hydrogen topara-hydrogen in a mixture thereof within the temperature range of fromabout 20 K. to about 240 K. which comprises raising the temperature ofthe mixture, passing said mixture through a finely dividedheat-activated bed of ferric oxide gel, and adding heat to replace theheat taken up by the endothermic reaction.

6. A process as set forth in claim 3, wherein the ferric oxide gelparticles are of a size which will pass a 20 mesh screen and theparticles are activated by heating to a temperature within the range offrom about 110 C. to about 140 C.

7. A process as set forth in claim 4, wherein the ferric oxide gelparticles are of a size which will pass a 20 mesh screen and theparticles are activated by heating to a temperature within the range offrom about C. to about C.

8. A process as set forth in claim 5, wherein the ferric oxide gelparticles are of a size which will pass a 20 mesh screen and theparticles are activated by heating to a temperature Within the range offrom about 110 C. to about 140 C.

9. The catalytic process for converting a mixture of ortho-hydrogen andpara-hydrogen at a temperature Within the range of from about 20 K. toabout 240 K. and below that at which said mixture is stable to a newmixture in a condition of stable energy equilibrium at said temperaturewhich comprises passing the unstable mixture through a finely-dividedheat-activated bed of ferric oxide gel particles while removing theexothermic heat of conversion therefrom.

10. The catalytic process for converting a mixture of ortho-hydrogen andpara-hydrogen at a temperature within the range of from about 20 K. toabout 240 K. and above that at which said mixture is stable to a newmixture in a condition of stable energy equilibrium at said temperaturewhich comprises passing the unstable mixture through a finely dividedheat-activated bed of ferric oxide gel particles while adding heat tomaintain the temperature and replace the endothermic heat of conversion.

;11. A process as set forth in claim 9, wherein the ferric oxide gelparticles are of a size which will pass a 20 mesh screen and theparticles are activated by heating to a temperature within the range offrom about 110 C. to about 140 C.

12. A process as set forth in claim 10, wherein the ferric oxide gelparticles are of a size which will pass a 20 mesh screen and theparticles are activated by heating to a temperature within the range offrom about 110 C. to about 140 C.

13. The catalytic process of converting a mixture of ortho-hydrogen andpara-hydrogen in an unstable energystate to an equilibrium energycondition at a temperature within the range of from about 20 K. to about240 K. comprising the steps of activating a finely divided ferric oxidegel by heating said gel particles to a temperature within the range offrom about 110 C. to about 140 C. in a vacuum and passing the unstablemixture through the thus activated ferric oxide gel particles while outof contact with the atmosphere.

14. A process as set forth in claim 13, wherein the gel particles are ofa size which will pass a 20 mesh screen and the passage of the unstablemixture through the ferric oxide gel particles is commenced while saidgel particles are still hot with the vacuum being broken by the passageof said mixture therethrough.

References Cited in the file of this patent UNITED STATES PATENTS370,511 Wigg et al. Sept. 27, 1887 1,008,321 Gill Nov. 14, 19111,812,527 Gross et al June 30, 1931 1,821,195 Woodhouse Sept. 1, @19311,904,439 Freyermuth Apr. 18, 1933 2,110,240 Roelen et al Mar. 8, 19382,465,235 Kubicek Mar. 22, 1949 OTHER REFERENCES Grilly: Atomic EnergyCommission, AECU-2245, November 1952.

1. THE PROCESS FOR CATALYTICALLY CONVERTING HYDROGEN TO AN EQUILIBRIUMENERGY STATE WITHIN THE TEMPERATURE RANGE OF FROM ABOUT 20*K. TO ABOUT240*K WHICH COMPRISES PASSING THE HYDROGEN THROUGH A POROUS BED OFHEATACTIVATED FERRIC OXIDE GEL PARTICLES.